So I was working on a picture of a wheat field in PS, when I thought, "You know what, lets see how this looks as my wallpaper." So I opened the image in Windows Picture and Fax Viewer, set as wallpaper, minimized, and WHAT?

This is probably a stupidly simple question, but the answer eludes me. Why would the image look different when set as wallpaper than it does in either Photoshop or Window's very own picture viewer?